Nitrogen oxides (NOx) are a well-known class of atmospheric pollutants that pose risks to human health, agriculture, and property. Nitrogen oxides are key ingredients in the formation of photochemical smog, which damages respiratory functions and limits visibility. Additionally, nitrogen oxides form nitric acid when they react with moisture in the atmosphere, thus forming acid rain. They are also involved in the production of tropospheric ozone. Environmental agencies around the world have instituted standards for nitrogen oxide control in an effort to reduce nitrogen oxides' harmful effects.
Nitrogen oxides broadly include compounds such as NO3 and N2O5, but the bulk of atmospheric nitrogen oxides are N2O, NO and NO2. When dealing with reactive atmospheric nitrogen, it is often sufficient to deal only with NO and NO2. (See Seinfeld and Pandis, Atmospheric Chemistry and Physics.)
While there are some natural sources of NOx, most of the NOx in urban environments is a by-product of fossil-fuel combustion. At temperatures found in engines, free-radical chemistry converts atmospheric nitrogen into nitrogen oxides. In particular, lean burning internal combustion engines, such as those used in diesel-powered vehicles, may produce NOx at a many parts-per-million level. While combustion processes typically produce more NO than NO2, the two easily interconvert in the presence of oxygen species. Thus, an effective emission control systems must account for both species. Additionally, NO2-producing processes present a greater health risk than NO-producing processes, as is reported in the literature.
In view of the health risks posed by atmospheric NOx, the U.S. Environmental Protection Agency regulates acceptable levels of NOx emissions for U.S. cars and trucks.